The control of flow for liquids and gases is a major problem in various manufacturing processes. Certain industries, notably the manufacture of silicon wafers, require very accurate gas flow rates, for many different gas species and with precise start flow and end flow points.
There have been a number of responses to the industry's need for accurate flow controllers. The prevalent type senses gas flow by measuring the thermal properties of gas flow over sensor elements. See for example, U.S. Pat. No. 4,658,855 to Doyle. The sensor flow measurement is combined with the control of a valve to proportionally enlarge or decrease the cross sectional area of the passageway through the valve and thereby control the flow of gas. If the sensor detects that the measured flow differs compared to the desired or set point flow, then a controller will partially close or open the valve until the measured flow equals the set point flow.
Although such control devices have resulted in acceptable products there is a need for a more universally applicable approach. The difficulty with the present methods arises because the thermal sensors used in such devices are quite dependent upon the heat transport properties of the gas species involved. The second is the difficulty associated with controlling the cross-sectional area of a valve opening for flow control. Minute changes in area result in substantial changes in flow imposing a need for a highly accurate method of valve opening. In practice, this means restricting the operations of the controllers to near their maximum displacement of the valve opening, which constitutes the smallest possible fraction of the valve area. Furthermore, for any given valve there is a relatively restricted range of flow over which changing the cross-sectional area is effective. The result of these restrictions is the widespread use of many different flow controllers each set up for a particular gas and maximum flow. However, they are largely non-interchangeable.
There are further difficulties inherent in the flow sensing concept of flow control. There has to be some flow to measure and this creates the "overshoot" problem wherein during the first few moments of flow after flow start, the actual flow will differ from the set point flow until the controller has had time to adjust the system. There is also the potential problem of oscillation wherein the inherent minimal adjustment of the controller might lag the measured flow. In such a situation the flow may oscillate between high and low states compared to the set point flow. Ideally this dampens out quickly to the set point flow but in any case magnifies the time period of the overshoot problem. In the worst case, oscillation drives the valve opening/closing sequence to a widely uncontrolled situation.
An objective of the invention was to provide an improved highly accurate flow rate controller, especially for small volumes of gas used in scientific, biomedical and engineering applications.